1. Field of the Invention
The present invention generally relates to optical communications systems, such as those used in telecommunications, and more particularly, to broadband or high speed digital communications systems of the type carrying information by the optical modulation of a light source via a surface plasmon wave device.
2. Description of Background Art
Optical communication systems are used with regularity today for many telecommunications systems. One such system of interest is the cable television system which is essentially a wired broadcast system, or point to multipoint system, which uses lightwaves over optical fiber cables to carry a broadband signal modulation. These systems modulate the laser light with 80-110 channels of programming with a signal which extends from about 50 MHz to about 1000 MHz or greater. In many systems, there is a reverse set channels in the band from 5 MHz to 50 MHz.
The optical communication systems of the cable television systems comprise a plurality of laser transmitters with a source frequency of either 1310 nm or 1550 nm which is modulated with the broadband signal which can be made up form a number of sources, such as satellite downlinks, terrestrial links, local programming, and UHF and VHF rebroadcasts. The modulated light is carried over optical fibers with perhaps some regeneration or amplification before being demodulated by an optical receiver at the other end of the optical fiber and thereafter distributed to a number of subscribers in a local geographic area around the receiver.
Because of the complexity of the broadband signal and its bandwidth, a common method for modulating the laser light is linear modulation of the intensity or amplitude of the light. This can be generally accomplished in one of two ways, either by direct modulation or by external modulation.
In direct modulation, a laser diode is biased with a current which is then mixed with the broadband signal to produce the intensity modulation. There are some problems with this technique including finding laser diodes with a linear current to light transfer characteristic. A very small percentage of predicable laser diodes can meet the stringent requirements for linearity, noise and output power which are needed in modern cable television systems. One solution has been to use less linear laser diodes but to compensate their nonlinearities by predistorting the broadband signal before direct modulation. While this concept has proven to be relatively effective, there are limits to the compensation as wider temperature range, operational power and bandwidth are required. Alternatively, external modulators, particularly of the Mach Zehnder (interferrometric) type have been utilized, where after a laser source signal has been generated, its optical output is modulated with a broadband signal applied to the external modulator. These systems however also produce nonlinearities in their transfer characteristics. In addition, there are inherent losses in the coupling of the source laser light to the modulator and its transmission through the modulator, thereby reducing effective operational power.
A surface plasmon wave (SPW) is an electromagnetic wave which propagates along the interface between two materials having dielectric constants of opposite signs, e.g., a metal and a dielectric layer. The polarization of a SPW is transverse magnetic (TM) and its electric field is perpendicular to the propagation interface. SPWs can be analyzed by techniques used for TM optical modes since they obey identical field equations and satisfy the same boundary conditions. Unique features of SPWs are that almost all of their energy is concentrated at the dielectric/metal interface and their propagation characteristics are very sensitive to environmental changes in their proximity.
Optical plasmon wave structures can be made by employing a device which couples an optical guided lightwave into a SPW. By controlling the amount of power coupling between the optical signal and the SPW, optical power may be dissipated producing an attenuation of the optical signal. If the amount of coupling is controlled electrically, a variable attenuator (modulator) device can be provided. Because of the extremely small interaction lengths needed, the optical plasmon wave modulator can be a very compact device which can be implemented as an integrated optics structure. This small size and other advantages of the device can produce substantial benefits with its use in the electronics and communications areas. One particularly advantageous plasmon wave modulator is disclosed in co-pending application Ser. No. 09/048,489, filed Mar. 26, 1998 in the name of E. Anemogiannis, and entitled " ", which application is assigned commonly with this present application. The disclosure of Anemogiannis is hereby incorporated by reference.